In a medium bowl, mix together the ground beef, egg, cracker crumbs, onion, salt, pepper and poultry seasoning using your hands. Shape into 6 patties about 1 inch thick.
Fry the patties in a large skillet over medium-high heat for 3 to 4 minutes per side, or until browned. Drain off grease, and remove patties to a platter; keep warm.
Melt the butter in the same skillet, and add the mushrooms. Cook and stir for about 2 minutes. Sprinkle the flour over, and mix in until blended. Stir in the milk and beef bouillon. Cook and stir over medium heat until smooth and starting to thicken. Return the patties to the gravy and cook over low heat, uncovered, for 10 minutes, stirring occasionally. (Add more flour for a thicker gravy)